1. Field of the Invention
The present invention relates to a semiconductor light emitting device emitting laser light in a stacking direction, and particularly relates to the semiconductor light emitting device suitably used in an application in which a large light output is desired.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) differs from an edge emitting laser diode in that the VCSEL emits light in a direction orthogonal to a substrate, and allows multiple devices to be arranged in a two-dimensional array on the same substrate. Thus, the VCSEL has attracted attention in the data-communication field in recent years.
A typical VCSEL includes, for example, a pair of n-type multilayer film reflecting mirror and p-type multilayer film reflecting mirror, and a cavity layer arranged between the pair of n-type multilayer film reflecting mirror and p-type multilayer film reflecting mirror on an n-type semiconductor substrate. The cavity layer includes an n-type cladding layer, an active layer including a light emitting region, a p-type cladding layer, and a current confinement layer in this order from the n-type multilayer film reflecting mirror side. The current confinement layer has an annular current confining region confining a current injecting region. The current confinement layer functions to improve current injection efficiency to the active layer, and to reduce threshold current. The pair of n-type multilayer film reflecting mirror and p-type multilayer film reflecting mirror have the configuration in which a low-refractive index layer and a high-refractive index layer with an optical film thickness of λ/4 (λ is a emission wavelength) are stacked alternately. The VCSEL is provided with an n-side electrode on a rear surface side, and a p-side electrode on a top surface side. The p-side electrode includes an aperture for emitting light from the light emitting region. In the VCSEL, current confined with the current confinement layer is injected to the active layer, and light generated in the active layer is reflected and amplified with the pair of n-type multilayer film reflecting mirror and p-type multilayer film reflecting mirror. As a result, the light is emitted from the aperture arranged in the p-side electrode.
In the above-mentioned VCSEL, the active layer is approximately a couple of tens of nm in thickness, and the light emitting region in the active layer is approximately 10 μm in diameter. The volume of the light emitting region in the VCSEL is much smaller than that of the light emitting region in an edge emitting laser diode. Thus, when the amount of current injected to the light emitting region increases, a light output is immediately saturated due to heat locally generated in the light emitting region. Therefore, simply increasing the amount of injected current does not contribute to increasing the light output. To avoid the immediate saturation of the light output, it is considered that an inner diameter of the current confining region in the current confinement layer (current confining diameter) is set larger so that the area (volume) of the light emitting region becomes larger. However, when the area of the light emitting region becomes large, multi-mode oscillation which destabilizes a distribution in the lateral direction of the light output is generated, and this leads to an issue that FFP (far field pattern) becomes unstable. Alternatively, it is considered that the area of the light emitting region is left as it is, and the thickness of the active layer is increased. For example, the active layer is configured with multiple quantum wells, and the active layer is thickened by increasing the stacking number of the quantum wells. However, when thickening the active layer in this way, current injection efficiency of each quantum well is reduced, and the threshold current is increased. Accordingly, there is an issue that even if the active layer is thickened, the light output is not so increased.
It is also considered that two active layers are provided within a cavity, and current is separately injected to each of the active layers. Thereby, without changing the amount of current injected to the active layer, the current confining diameter, or the thickness of the active layer, the thickness of the active layer as seen from the whole VCSEL may be increased. Therefore, the light output may increase while FFP is stabilized. For example, Japanese Unexamined Patent Publication No. 2007-227860 discloses a following technique. Between a pair of p-type multilayer film reflecting mirror and an n-type multilayer film reflecting mirror, a PIN structure by stacking a p-type current confinement layer, a p-type cladding layer, an active layer, and an n-type cladding layer in this order from the p-type multilayer film reflecting mirror side, and a PIN structure by stacking an n-type cladding layer, an active layer, a p-type cladding layer, and a p-type current confinement layer in this order from the n-type multilayer film reflecting mirror side are provided. Moreover, an n-type contact layer is provided between these PIN structures. An electrode electrically connected to this n-type contact layer is regarded as a common electrode for both of the PIN structures, and current is injected in parallel from the common electrode to the two active layers. Thereby, current is separately injected to the two active layers.